Current trends in the electronics industry are moving toward higher signal, power, and ground line densities, smaller size packages, and increased performance characteristics, such as less crosstalk, lower inductances, and greater resistance to failure from thermal cycling stress. The higher signal line densities in electronic packages may require interconnection leads on less than 0.6 mm centers or pitch. Typically these would be fabricated using fine pitch technology versus surface mount or through hole technologies. One type of fine pitch technology is tape automated bonding (TAB). TAB uses photoimaging and etching processes to produce conductors on a dielectric/conductor tape which looks similar to movie film formats. The most visible aspect of a TAB system is the carrier tape, which typically is 8 to 70 mm wide and stored on reels. The tape is formed by an insulating substrate, typically polyimide, which can have a single side or both sides with a thin conductor layer, usually copper. At specific locations in the tape there are windows or cavities through the substrate and conductor layers. A conductor layer contains the signal pattern to give the desired interconnection circuitry with `beam-type` leads or fingers that extend over the windows in the tape. In subsequent processing, the beams are bonded to an integrated circuit die or chip that is precisely located under the windows. Sprocket holes or features on the tape carrier permits automated tape-handling equipment to accurately position the individual dice for the subsequent processing operations, such as testing, burn-in, and mounting.
There are many advantages to using a tape automated bonding system to mount semi-protected integrated circuit dice to printed boards. Some advantages of TAB are that it provides; a means for pretesting and `burn-in` of the dice prior to their final mounting, the possibility of automated mass bonding of the leads to dice, improved heat transfer versus wire-bonded attachment due to the copper beam leaded technique, and many more which one skilled in the art would appreciate. Because of the many advantages of tape automated bonding, the manufacturers of components for the electronics industry have considerable incentive to overcome the disadvantages and technical problems in the formation of the TAB carrier tape.
One such problem is the proper formation of the beam-type leads or fingers of the signal layer which extend into windows in the tape. These leads are very fine with typically less than 0.003 in. (0.0762 mm) in width, 0.0014 in. (0.036 mm) in. thickness, and 0.030 in. (0.762 mm) in length. Also once the leads are formed, the substrate no longer supports them from underneath, so that the critical stress point for the leads is located at the periphery of the window where the substrate support ends and the base of the beam-type lead begins. It is desirable at this location to have the peripheral wall of the window as close to perpendicular to the beam-type leads as possible, by minimizing the undercutting of the substrate in the window. The leads must be adequately supported by the substrate at the peripheral wall of the window in order to prevent the fine lead from bending and shorting the die once attached or in the case with 2 conductor layers, shorting with the opposite conductor layer.
It is therefore an object of this invention to provide a method to minimize the undercut of a polyimide substrate at a location where fine pitch beam-type leads are formed and the terminus of such lead extends unsupported into a window cavity.